This question is more general and doesn't apply specifically to ecommerce sites even though the discussion mentions them a little.
Specifically, we know we can't do business on Shabbat. So what is the action on the part of the user and on the part of the owner that is considered "doing business"? Certainly that would be forbidden?
And if, for example, it's when the money becomes available, why doesn't that forbid me from doing business in the days leading up to Shabbat (depending on how long it takes the funds to become available)?